1. Field of the Invention
The present invention generally pertains to molecular electronics and more specifically to the formation of metal contacts connected to molecules and molecular films.
2. Description of the Background
Nearly all modern electronic devices are based on the complementary metal-oxide-silicon field-effect transistors (CMOS-FET). Manufacturing improvements of solid-state devices, such as the CMOS-FET, have nearly doubled computing powers every eighteen months for the past thirty years. These improvements are a direct result of the miniaturization of devices used in computer processors. Unfortunately, CMOS technology is beginning to show limits associated with the fundamental physical laws governing device performance and technical problems associated with manufacturing. Thus, alternative technologies are being sought which are unlike FET devices that operate based upon the movement of electrons in bulk material. These new technologies operate in the realm of quantum mechanical phenomena that emerge and dictate electron dynamics at the nanometer (10xe2x88x929 m) scale.
A specific area that has shown considerable promise in the miniaturization of computer processor devices is a component technology that uses individual molecules or arrays of molecules, that is termed molecular electronics. This relatively new approach exploits the fact that molecules are naturally occurring nanometric structures. By devising molecular structures to act as electrical switches, then by combining these switches into complex circuit structures, computational nanocircuitry can be achieved. In this manner, conductive electrodes are attached to a molecule within a layer of molecules, and thus the layer may perform functions in an analogous fashion to a solid-state device. The application of molecular electronics greatly reduces the scale of individual devices to nanometers per device; therefore, more than a billion devices may be contained per square centimeter.
A typical molecular electronic device is composed of two or more contacts (sometimes called gates or terminals) and a molecular film or a molecule that is attached to these contacts. The fabrication procedure involves forming a molecular film (MF), which can be as thin as one molecular layer, on one contact. Additional contacts are then attached to the molecule. These devices function by modulating electron flow between the contacts through the MF. The flow of current through the molecule and performance of the device are specifically dependent upon the chemical and molecular structure of the molecule and the strength of the interaction between the molecular layer and each of the contacts. In most cases, the first contact formed in these devices is a covalent chemical bond formed in solution. The second contact must be formed on top of the MF.
In order for the molecular electronic device to function properly, each of the contacts must be electrically isolated from one another. Achieving this isolation is complicated by the length scale between contacts, which is defined by the length of the molecule (1 nm-5 nm). To ensure electrical isolation, and long-term performance of the device, a strong interaction between the metal contacts and the MF is required. Additionally, this strong interaction between the molecule and the contacts eliminates some unwanted device characteristics (e.g. device shorting).
Presently, metal contacts in molecular electronic devices are formed by evaporating a metal layer to the surface-bound MFs. This technique is performed within a vacuum chamber and is called vapor deposition. The interaction between the evaporated metal layer and the molecule is highly dependent on the metal and the chemical composition of the MF. This interaction between the metal and a MF may be changed through the introduction of a functional group to the molecular structure. For example, amine or carboxylic acid functional groups decrease the extent of metal penetration through a MF when metal contacts are grown on top of the monolayer by evaporation that can cause degradation or shorting of the electrical contacts.
The present invention overcomes the disadvantages and limitations of the prior art by providing a method for attaching metal contacts to individual molecules and/or aggregates of molecules that form a molecular film by using electroless deposition (ELD) to form a metal contact on a MF. ELD is a solution-based technique where a metal is catalytically reduced at a surface.
The present invention may therefore comprise a method of attaching metal contacts to functional groups on individual molecules in a molecular film to facilitate their function as solid-state devices comprising: attaching a functional molecular group to selective portions of a metallic plate to form patterned areas of self-assembled monolayers on a conductive bottom contact, patterning the functionalized molecules on a surface to give spatial control over the location of the metal contacts, placing the patterned surfaces of the self-assembled monolayers into an electroless plating bath for selective metal deposition onto the surfaces, controlling the chemical state of the functional molecular group by preferentially inducing the electroless metal deposition at the functional molecular group site, attaching metal contacts to the patterned portions of the self-assembled monolayers to form a top contact by depositing a metallic layer on the non-metallic surfaces of the self-assembled monolayers with an electroless plating bath consisting of water, formaldehyde, copper sulfate, sodium hydrogen tartrate, adjusting the pH of the electroless plating bath to approximately 12.8 using sodium hydroxide, removing the self-assembled monolayers from the electroless plating bath after a prescribed length of time and rinsing the self-assembled monolayers with water.
The present invention may also comprise a solid-state device made by the process comprising: attaching a functional molecular group to a selective portions of a metallic plate to form patterned areas of self-assembled monolayers on a conductive bottom contact, patterning the functionalized molecules on a surface to give spatial control over the location of the metal contacts, placing the patterned surfaces of the self-assembled monolayers into an electroless plating bath for selective metal deposition onto the surfaces, controlling the chemical state of the functional molecular group by preferentially inducing the electroless metal deposition at the functional molecular group site, attaching metal contacts to the patterned portions of the self-assembled monolayers to form a top contact by depositing a metallic layer on the non-metallic surfaces of the self-assembled monolayers with an electroless plating bath consisting of water, formaldehyde, copper sulfate, sodium hydrogen tartrate, adjusting the pH of the electroless plating bath to approximately 12.8 using sodium hydroxide, removing the self-assembled monolayers from the electroless plating bath after a prescribed length of time and rinsing the self-assembled monolayers with water.
The disclosed embodiments have numerous advantages over prior art. These include the elimination of additional process steps required to make the MF compatible with a vacuum environment and greater control over the chemical state of the MF. With the present embodiment, the chemical state of the MF can be readily varied in solution by changing variables such as pH and ionic strength. The increased control gained by producing the MF in solution greatly increases the type of physiochemical interactions that may be formed between the metal contact and the molecular layer. Additionally, ELD offers spatial control and precision at the required device densities necessary in molecular electronic applications. These advantages make the present embodiment suitable for applications in a variety of areas including, but not limited to, molecular electronics, electrical contacts, metal patterning and electroless deposition.
Numerous advantages and features of the disclosed embodiments will become readily apparent from the following detailed description of the invention and the embodiment thereof, from the claims and from the accompanying drawings in which details of the invention are fully and completely disclosed as a part of this specification.